Field
The described embodiments relate generally magnetic resonance (MR), more specifically to characterizing tissue based on one or more medical resonance techniques, such as magnetic resonance imaging (MRI), magnetic resonance spectral imaging (MRSI) and/or magnetic resonance fingerprinting (MRF).
Related Art
Magnetic resonance or MR (which is often referred to as ‘nuclear magnetic resonance’ or NMR) is a physical phenomenon in which nuclei in a magnetic field absorb and re-emit electromagnetic radiation. For example, magnetic nuclear spins may be partially aligned (or polarized) in an applied external magnetic field. These nuclear spins may precess or rotate around the direction of the external magnetic field at an angular frequency (which is sometimes referred to as the ‘Larmor frequency’) given by the product of a gyromagnetic ratio of a type of nuclei and the magnitude or strength of the external magnetic field. By applying a perturbation to the polarized nuclear spins, such as one or more radio-frequency (RF) pulses (and, more generally, electromagnetic pulses) having pulse widths corresponding to the angular frequency and at a right-angle or perpendicular to the direction of the external magnetic field, the polarization of the nuclear spins can be transiently changed. The resulting dynamic response of the nuclear spins (such as the time-varying total magnetization) can provide a wealth of information about the physical and material properties of a sample.
In medicine, MR has been widely used to non-invasively determine anatomical structure and/or the chemical composition of different types of tissue. For example, in magnetic resonance imaging (MRI), the dependence of the angular frequency of precession of nuclear spins (such as protons or the isotope 1H) on the magnitude of the external magnetic field is used to determine images of anatomical structure. In particular, by applying a non-uniform or spatially varying magnetic field to a patient, the resulting variation in the angular frequency of precession of 1H spins is typically used to spatially localize the measured dynamic response of the 1H spins to voxels, which can be used to generate images of the internal anatomy of the patient. Alternatively, in magnetic resonance spectral imaging (MRSI) the measured dynamic response of other nuclei in addition to 1H are often used to generate images of the chemical composition or the morphology of different types of tissue and the internal anatomy of the patient.
Typically, existing MR techniques such as MRI or MRSI are used to measure a limited set of physical or material properties. Moreover, these MR techniques usually provide qualitative or ‘weighted’ measurement of these properties. In particular, the MR signal intensity is rarely quantitative by itself. Instead, analysis of MR signals often involves relative comparisons of spectral peaks, spatial locations or different points in time.
Recently, researchers have used MR to measure multiple parameters simultaneously and to provide quantitative measurements of sample properties. In particular, instead of using repeated, serial acquisition of data to characterize individual parameters that are of interest, in magnetic resonance fingerprinting (MRF) signals from different materials or tissues are usually acquired using a pseudorandom pulse sequence to determine a unique signal or ‘fingerprint’ (e.g., a time-dependent magnetization or MR trajectory) that is a function of multiple material properties under investigation. In principle, using pattern-recognition techniques the quantitative multi-parameter fingerprint can be matched to predefined states (such as the presence of a particular disease) and can improve measurement accuracy.
Because the spatial resolution of MR techniques usually depends on the magnitude of magnetic field gradient, there are ongoing efforts to increase the magnetic field strength, e.g., using superconductors. However, the use of large magnetic-field strengths usually increases the size and cost of an MR scanner.
In addition, because of measurement variation from scanner to scanner, and even among repeated measurements performed by the same scanner, it has proven difficult to perform reliable or reproducible quantitative MR measurements. Consequently, in spite of the wide-spread use of MR in medicine, the true potential of this powerful measurement technique still has not been achieved, which can be frustrating to healthcare providers and can adversely impact patient outcomes.